Effects of silica on chemical recovery in the direct causticization of wheat straw black liquor

Straw black liquor is relatively difficult to recover using the Tomlinson process because of the presence of silica in inorganic components. Gasification with direct causticization might serve as a promising alternative; however, the interaction between silica and direct causticization agents needs to be evaluated. In this work, desilication was achieved using a laboratory-scale membrane electrolysis cell adopted from the chlor-alkali industry. Then raw and desilicated straw black liquors were carried out through direct causticization with titanium dioxide (TiO2) and recycled sodium tri-titanate (Na2O·3TiO2) at 850°C using a tube furnace. The results show that these two agents exhibit different silica transfer mechanisms. When using TiO2, approximately 80% of the silica is retained in the solid residue after hydrolysis because of chemical interactions. Almost all of the silica transfers to the hydrolysis-derived white liquor when using Na2O·3TiO2. This means that the process of treating straw black liquor by direct causticization might need additional lime causticization to improve the efficiency of sodium recovery. This observation also indicates that silica will not consume the direct causticization agent Na2O·3TiO2 after the initial step using TiO2. The sodium hydroxide (NaOH) yield was 60% when using raw black liquor as the feed to the Na2O·3TiO2 recycling tests and 80% when using desilicated straw black liquor. Considering the NaOH recovered in the electrolysis cell, the total NaOH yields exceed 85% of total titratable alkali. No detectable deterioration of the direct causticization agents was found within the limited number of recycles tested. A greater number of cycles need to be tested at a larger scale to evaluate the feasibility of applying a direct causticization process to straw black liquor.